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ESXi and Lopsided Bootbanks

While working on a white paper that discusses everything you ever wanted to know about upgrading to ESXi 5.0 I ran across this interesting quote in the vSphere 5.0 Upgrade Guide:

For most ESXi 4.x hosts, the partition table is not rewritten in the upgrade to ESXi 5.0. The partition table is rewritten for systems that have lopsided bootbanks. Lopsided boot banks can occur in systems that are upgraded from ESXi 3.5 to ESXi 4.x, and then upgraded directly to ESXi 5.0.

I really like the term “lopsided bootbanks” because it just sounds cool.  However, besides trying to figure out if the correct term is “bootbanks” or “boot banks” this also made me curious about what exactly are lopsided boot banks, how do I know if I suffer from this condition, and what is the implication of having lopsided boot banks?  I did some research and here’s the scoop on lopsided boot banks.

What are Lopsided Boot Banks

ESXi implements a dual-image architecture wherein the boot disk includes two partitions on which the ESXi image is stored; these partitions are referred to as boot banks.  One boot bank will have the active  image, referred to as the primary boot bank, and the other will have the previous image, referred to as the alternate boot bank.  Anytime you update your image or deploy a new image, the active image is copied from the primary boot bank to the alternate boot bank and the new image is installed into the primary boot bank. 


This dual-image approach is pretty slick as it enables quickly falling back to a previous image should a problem develop following a patch or update.  You simply reboot the host and press “shift+R” at the beginning of the boot process to instruct the boot loader to boot off the alternate boot bank.  This dual image approach provides a built in rollback capability when updating your ESXi hosts.

Where lopsided boot banks come into the picture is when upgrading from ESXi 3.5 to ESXi 4.x.  In ESXi 3.5 the size of the boot banks is 48MB.  In ESXi 4.x the size was changed to 250MB.  However, when a host is upgraded from ESXi 3.5 to ESXi 4.x only the size of the primary boot block is grown from 48MB to 250MB.  The size of the alternate boot block is left at 48MB.  This condition of having one boot bank larger than the other is referred to as having “lopsided bootbanks”.


Having lopsided boot banks is a temporary condition.  Remember the purpose for having two boot banks is to maintain a “last known good copy” of the ESXi image.  As such, the first time you apply a patch or update to an ESXi 4.x host with lopsided boot banks, the alternate boot bank will also get resized to 250MB, and hence you will no longer have lopsided boot banks. 

As such, the lopsided boot banks issue only applies when you have an ESXi 3.5 host that is upgraded to ESXi 4.x and then immediately upgraded to ESXi 5.0.  If you upgraded an ESXi 3.5 host to ESXi 4.x several months back and have since applied updates to the host, the lopsided boot banks issue will not apply when upgrading to ESXi 5.0.

Checking for Lopsided Boot Banks

How do I check for lopsided boot banks?  Probably the easiest way to check for lopsided boot banks is from the vSphere client.  From the Hosts and Clusters view select the ESXi Host -> Configuration -> Storage.  From the list of datastores select the boot disk, and then from the Datastore Details section select properties.  In the Extent Device section on the bottom right look at logical partitions 5 and 6.  If one shows 48MB and the other 250MB you have lopsided boot banks.

Here’s a screen shot showing an ESXi 4.x host with lopsided boot banks:


For reference, here’s a screen shot showing an ESXi 4.x host where both boot banks are 250MB:


Implication of Lopsided Bootbanks

The implication of having lopsided boot banks is really pretty minor.  It's largely transparent and the only limitation is that you won’t be able to use Update Manager to upgrade the host to ESXi 5.0.  Update Manager assumes the boot banks are both at 250MB.  If it detects one boot bank is smaller than the other it will report an invalid boot disk and won’t even try to upgrade.  If this happens you can still upgrade the host using the ESXi 5.0 Installer.   Another option would be to first apply some patches/updates to the host to resize the alternate partition and then upgrade the host using Update Manager.


So in the end, despite having a very cool name, having "lopsided bootbanks" is really not that big of deal.  Because it only applies to host that are upgraded from ESXi 3.5 to ESXi 4.x and then immediatly to ESXi 5.0 it will be an exception rather than the norm.  Nonetheless, it's good to know.  

Oh, and the white paper should be available around the same time 5.0 officially releases.